This invention relates to a contrast medium (imaging agent) used for medical ultrasonic diagnosis.
Ultrasonic diagnosis uses acoustic physical phenomena that an ultrasonic wave having a short wavelength travels almost straight in a living body as a slender beam-like sonic wave, and while this beam-like ultrasonic wave travels in living tissues, a part thereof reflects, scatters, decays due to absorption or is influenced by the Doppler effect at the boundary surface of organs or tissues having different acoustic characteristics. Changes of acoustic characteristics are detected by an ultrasonic probe.
In ultrasonic diagnosis of a living body, an ultrasonic contrast medium is used in order to obtain information on cardiac and vascular bloodstreams and a urine stream. This contrast medium is injected parenterally into a circulatory system and moved to an organ to be contrasted, so that it is required that the contrast medium does not cause any damage to a living body, is stable and exhibits good dispersibility in a fluid to be tested.
As an ultrasonic contrast medium which satisfies such requirements, there has been disclosed a dispersion of microspheres having a diameter of 10 .mu.m or less in which fine bubbles are encapsulated in a substance having affinity for an organism obtained by insolubilizing a thermally sensitive protein such as albumin in water in U.S. Pat. No. 4,844,882 and U.S. Pat. No. 4,957,656.
The above contrast medium comprising hollow microspheres in which gas is encapsulated has a large contrast effect, but has problems that the contrast medium cannot be introduced centrally in the vicinity of a site to be observed in a living body, so that an excessive amount of the contrast medium is required, and further, specific gravity of the medium cannot be controlled, so that it cannot be used for observation of a fluid having a slow flow rate such as urine.